Can a MaxxMacro 54 master be used on a MaxxMacro 70 chuck?

Yes. The system is downward compatible, meaning MaxxMacro (System 3R OEM 100% cross-compatible) 54 fixtures work on MaxxMacro 54, MaxxMacro 70, and MaxxMagnum chucks; MaxxMacro 70 fixtures work on MaxxMacro 70 and MaxxMagnum chucks; and MaxxMagnum fixtures work only on MaxxMagnum chucks.

That said, regarding masters, we recommend using the same system size.